This disclosure relates to a coating for components in applications such as gas turbine engines and internal combustion engines.
Many gas turbine engine components are subject to temperatures in excess of the melting temperature of the component substrate, which may be constructed from a nickel superalloy or non-oxide ceramic, for example. Cooling features and thermal barrier or environmental coatings are used to protect the substrate from these extreme temperatures.
Thermal barrier coatings (TBC) or environmental barrier coating (EBC) made from yttria-stabilized zirconia (YSZ) and gadolinium zirconium oxide are typically used to reduce the temperature of cooled turbine and combustor components. Additionally, these materials may also be used as abradable seal materials on cooled turbine blade outer air seals (BOAS) as well as other components. In these applications, there are several degradation and failure modes. During engine operation, thermal barrier coatings may become spalled, delaminated, chipped or eroded, for example, due to debris or environmental degradation.
Bond coats for thermal barrier coatings as well as environmental barrier coatings for high temperature composite materials rely on an oxygen diffusion barrier layer that is often alpha alumina. Alpha alumina is an excellent barrier to oxygen diffusion and naturally forms at elevated temperature on aluminum rich alloys. This layer is often referred to as a thermally grown oxide (TGO). As this TGO layer grows with time and temperature it typically buckles and spalls off at which time it must regenerate, drawing further from the aluminum reserves of the host material by diffusion. As the aluminum gets depleted, non-ideal oxides begin to form which make the TGO less effective at preventing further oxidation. Making TGO adhesion more difficult is the coefficient of thermal expansion (CTE) mismatch between the metallic aluminum donor and alumina layers during the thermal cycling present in most applications.